We have rotating banners on our sites (it put colors)... advertizing ourselves mainly, 2 or 3 CJ and friends - all for free. The best click ratio is 5.1%, the lowest one is 1.413%. I'll say that the average is around 2.5% as there are plenty of 3%. Definiively the 5.1% is for casino hotels, the 1.413% is for an electronic store and we are travel agents... relevancy one more time.
Banners can still work for both advertisers and website publishers. As someone pointed out, if they weren't working they'd entirely disappear. Sure, they don't work as well as some other ad formats, but I've used them very effectively recently as both an advertiser to generate conversions and as a website publisher to make revenue.
It pays off, but I think it depens of how much people actually view it. The more website traffic, the more it pays off, of course. It's no use when you have it on a site with very low traffic however.